During or after the drilling of oil or gas wells, measurements of the electrical characteristics of the wellbore are performed. Electrical earth borehole logging is well known and various devices and techniques have been described. A variety of measurements may be made, but typically include resistive measurements extending deep into the formation and also superficial measurements of changes in resistivity at the surface of the borehole. In an electrical investigation of a borehole, current from an electrode is introduced in the formation from a tool inside the borehole. If this current is maintained constant, the voltage measured at a monitor electrode is proportional to the resistivity of the earth formation being investigated. If the current is varied to maintain constant the voltage measured at a monitor electrode, the current is inversely proportional to the resistivity of the earth formation being investigated. If both voltage and current are allowed to vary, their ratio is proportional to the resistivity of the earth formation being investigated. Substantial advances have been made in such electrical investigations by using electrodes whose currents are focused by other electrodes and thus determine the resistivity of the formation at a desired distance from the borehole wall surface. Examples of such techniques and devices for focused electrical investigation are described and shown in the U.S. Pat. No. 2,712,629 to Doll; U.S. Pat. No. 2,750,557 to Bricuad; U.S. Pat. No. 3,521,154 to Maricelli; and U.S. Pat. No. 4,468,623 to Gianzero et al.
In U.S. Pat. No. 2,712,629 to Doll, pad mounted sets of electrodes are described as each formed of a central survey electrode surrounded at spaced intervals by continuous guard electrodes embedded in segmented recesses.
In U.S. Pat. No. 2,750,557 to Bricuad, the pad mounted electrodes are formed of electrically directly connected segments or buttons.
In U.S. Pat. No. 3,521,154 to Maricelli, a plurality of survey electrodes are mounted on a single pad as a composite focusing electrode, with a pair of the survey electrodes aligned along the direction of travel of the tool along the borehole and one survey electrode horizontally displaced to provide a technique for effectively improving the signal to noise ratio of the resistivity measurements.
In U.S. Pat. No. 4,468,623 to Gianzero et al., an earth formation investigating tool is described in which borehole wall features on the order of millimeters in size can be detected. The tool includes an array of small cross-section survey electrodes (buttons) which are pressed towards the borehole wall and each button injects an electric current into the adjoining formation. The individual button currents are monitored and signals representative of button currents are recorded as curves as a function of depth. The measured button currents reflect the resistivity of the material in front of each button. In order to achieve a high resolution investigation, the electrodes are arranged in an array of multiple rows. The electrodes are so placed at intervals along a circumferential direction about the borehole axis as to inject survey currents into borehole wall segments which overlap with each other to a predetermined extent as the tool is moved along the borehole wall. In this manner, a detail high resolution resistivity or conductivity investigation of the borehole wall can be made. The presence of a fracture may be identified by noting a deviation between the survey currents from different pads. Such survey current deviation may indicate a fracture by virtue, for example, of the invasion of higher conducting mud into the fracture.
As inferred from the foregoing reference to "higher conducting mud", such logging tools primarily were designed for use in an electrically conductive aqueous-based fluid. Accordingly, it has been possible to obtain electric logs from well boreholes primarily by suspending the logging tool in an electrically conductive aqueous media. As oil wells were and continue to be drilled deeper into water sensitive formations, the application of non-aqueous-based drilling fluids or invert emulsions of water or brine in the various types of fluids used in such non-aqueous-based drilling fluids, such as organic solvents, diesel fuel, mineral oil, vegetable oil and synthetic fluids, is increasing. Because the continuous phase of this invert emulsion is usually a non-conductor of electricity, conventional electric logs which require passage of electrons through a conductive media have not been effective in these types of drilling and completion fluids.
Some logging tools have been designed for use in oil-based drilling fluids. For example, in such tools, knife-edge electrodes have been used to ensure contact with the filter-cake or mudcake which usually forms on the side of the borehole. However, U.S. Pat. No. 3,521,154 to Maricelli notes that since the oil-based drilling fluid or mud is relatively non-conductive, even the slightest separation between the knife-edge and the mudcake will provide erroneous indications of the conductivity of the adjoining formation. In U.S. Pat. No. 2,930,969 to Baker, the tool thereof may employ brush-like contacts which scratch through the mudcake and effect good electrical connection with the rock when an oil-based fluid is used.
A few attempts to make oil-based drilling fluids electrically conductive for the purpose of electrical logging have been reported though none of them has been a commercial success. U.S. Pat. No. 2,696,468 to Fischer disclosed a conductive oil-based drilling fluid containing up to 10 percent by weight water, an electrolyte and certain types of emulsifying agents, specifically sulfated and sulfonated organic compounds which promote the formation of oil-in-water emulsions. The electrolytes were water-soluble ionizable metallic compounds and were for the most part water-soluble salts of alkali- and alkaline-earth metals and alkali-metal hydroxides. Though almost forty compounds, including magnesium chloride, magnesium nitrate and magnesium sulfate, were specifically named as electrolytes, only seven sodium-containing compounds and calcium chloride were exemplified. Fischer disclosed a particular preference to alkali-metal hydroxides, silicates and phosphates. U.S. Pat. No. 2,739,120 also to Fischer discloses similar oil-based fluids which are asserted to be electrically conductive and contains a non-ionic surfactant rather than the emulsifiers of U.S. Pat. No. 2,696,468. Though almost forty compounds, including magnesium chloride, magnesium nitrate and magnesium sulfate, are specifically named as electrolytes, only sodium-containing compounds were exemplified. Both of these patents disclosed that in order to maintain the general desirable characteristics of oil-based drilling fluids, the water content should be maintained below 10 percent by weight, i.e., avoiding the formation of a water-in-oil or invert emulsion.
About twenty-five years later, Hayes et al. in U.S. Pat. No. 4,012,329 disclosed water-in-oil microemulsion drilling fluids which were asserted as being capable of conducting electrical current and as such permitted the use of ordinary electrical logging techniques. This fluid contained water, sodium petroleum sulfonate, hydrocarbon, bentonite and, optionally, cosurfactant, electrolyte, gelling agents and fluid loss agents. The electrolyte was a water-soluble inorganic base, inorganic acid or, preferably, inorganic salt. Certain sodium and potassium salts were identified, but not specifically identified in the examples.
Therefore, a need exists to modify non-conducting fluids in order to effectively use the great variety of conventional electrical well logging tools, particularly imaging tools.